Gas turbines are well known and used in various applications. As illustrated in FIG. 1, a typical gas turbine engine 10 includes a compressor 12 which draws in ambient air 14 and delivers compressed air 16 to a combustor 18. A fuel supply 20 delivers fuel 22 to the combustor 18 where it is combined with the compressed air to produce high temperature combustion gas 24. The combustion gas 24 is expanded through a turbine 26 to produce shaft horsepower for driving the compressor 12 and a load such as an electrical generator 28. The expanded gas 30 is either exhausted to the atmosphere directly, or in a combined cycle plant, may exhausted to atmosphere through a heat recovery steam generator (not shown).
The fuel flow supplied to the combustor 18 from the fuel supply 20 will vary with variations in the operating condition of the engine 10, such as in the range of operation from ignition to full load. For example, in gas turbines fueled by a fuel oil, the fuel flow to the combustor 18 may be controlled with reference to a differential pressure at a fuel nozzle located with in the combustor 18 to ensure that proper fuel atomization occurs throughout the operating range of the engine.
In a known fuel delivery configuration, the pilot nozzles in a dry low NOx combustion system comprise a duel nozzle structure including a primary nozzle, defining a primary stage, and a secondary nozzle, defining a secondary stage. At lower loads and low fuel flow rates, all fuel is injected into the combustor through the primary stage, providing good atomization of the fuel. At higher loads, the fuel is injected through both the primary and the secondary stages to provide the required flow volume at moderate pressures. Specifically, in a known construction of a dual nozzle structure, a spring-loaded valve is provided in a fuel line between the primary and the secondary nozzles. As long as the differential pressure between the fuel supply pressure and the pressure in the combustion zone of the combustor is below a threshold value, the valve remains closed and all fuel flow goes through the primary stage. As the supply pressure increases, the fuel flow through the primary stage increases until the crack pressure of the valve is reached, and the valve opens to allow fuel flow to the secondary stage. The pressure differential for driving atomization of the fuel in the secondary stage is equal to the differential between the supply pressure and the combustion zone pressure, minus the crack pressure of the valve. Since this pressure differential at the secondary stage is very low just above crack pressure, i.e., just after the valve opens, the atomization of fuel injected through the secondary stage is typically less than optimum at this operating point.
In addition to the above-mentioned problems, pressure actuated valves may become stuck in either an open or closed position, and may experience a condition called “chatter” where the valve opens and closes rapidly in the operating region of the crack points, which may produce undesirable dynamics in the combustor.
FIG. 2 illustrates the flow characteristic curve for known pilot nozzles and depicts simplex (single nozzle) and pressure actuated duplex (dual nozzle) approaches. Line 4 illustrates the simplex nozzle flow where it is necessary to provide a high enough flow to meet base load flow requirements, resulting in less than optimum atomization at lower pressures. Two duplex approaches are also illustrated in FIG. 2, including different crack pressures, one at 600 psi and the other at 1000 psi. Line 6 depicts a first duplex approach in which the flow number ratio (secondary nozzle/primary nozzle) is 2:1. The flow condition depicted by line 6 comprises a crack pressure of 600 psi (point 5), where the secondary flow is initiated just before a full-speed-no-load (FSNL) condition. It may be seen that this is not desirable in that nozzle “chatter” may be a problem when idling at FSNL. Line 8 depicts a second duplex approach in which the crack pressure is increased to 1000 psi (point 7) which, while moving the line slightly above FSNL, may still be too close to FSNL to avoid problems in that the flow is not precisely known. As with the first approach, the pressure actuated valve providing the secondary flow will be subject to “chatter.” Additionally, the flow number of the secondary nozzle in the second approach would need to be almost twice that of the secondary nozzle in the first approach in order to meet the base load fuel requirements, providing less than optimum atomization.